Flame-resistant polycarbonates have in the past been prepared industrially preferably by the incorporation of tetrahalogenated dihydric phenols, for example of tetrachlorobisphenol A or tetrabromobisphenol A. Although grading in a category of high flame resistance (for example v-o according to Underwriters' Laboratories, Subj. 94) can be achieved in this way, this process effects a drastic impairment in the processability of the corresponding polycarbonates, which is caused by the reduced flow properties and reduced heat stability of these polycarbonates.
Polycarbonates can also be rendered flameproof by admixing homopolycarbonates or copolycarbonates based on halogenated bisphenols, but this process also does not give molding compositions which have good processability combined with good flame-resistance.
Mixtures of an aromatic homopolycarbonate with a low-molecular weight homopolycarbonate obtained from tetrahalogenated bisphenols, such as, for example, tetrabromobisphenol A, are also known. Products of this type also display greatly reduced flowability and reduced heat stability, which have an adverse effect on their behaviour or processing in injection molding machines (for literature, see, for example U.S. Pat. No. 3,334,154 and DT-OS (German Published Specification) No. 2,243,226).
Even compared with the halogenated polycarbonates according to DT-OS (German Published Specification) No. 2,315,888 and U.S. Pat. No. 3,912,687, incorporated herein by reference, and their admixtures with other polycarbonates, for example bisphenol A polycarbonates, the polycarbonates according to the present invention show improved intrinsic viscous properties associated with equal or better flame-resistance.
Thus, to summarize it can be stated that customary halogen-containing polycarbonates themselves, and their admixtures with halogen-free polycarbonates, certainly show improved behaviour in burning, but have processing disadvantages due to the reduced flowability.